Apparatus and method for NOx reduction in natural gas furnaces

ABSTRACT

An improved combustion system for reducing NOx in a furnace  10  is provided having a primary flame zone  12.  The primary flame zone  12  includes a plurality of burners  14  located therein which are operated on a sub-stoichiometric level thereby producing uncombusted natural gas (or hydrocarbon). The burners  14  also facilitate the passage of the uncombusted natural gas (or hydrocarbon) from the primary flame zone  12  into a secondary flame zone  18  above the primary flame zone. The secondary flame zone  18  is provided with a natural draft port  20,  having a predetermined cross-sectional area, formed therein at a predetermined distance above the primary flame zone  12.  As a result, air of a predetermined volume is induced into the secondary flame zone  18  so that uncombusted natural gas (or hydrocarbon) in the secondary flame zone is combusted thereby reducing NOx emissions from the furnace  10.

TECHNICAL FIELD

[0001] This invention relates to Natural Gas Furnaces, and more particularly to an apparatus and method for reducing Nitrogen Oxide (NOx) emissions in natural gas furnaces. When fuel is combusted in a furnace oxygen from the air combines with nitrogen from the fuel or air to produce nitrogen oxides. NOx of course is one of those components that combine with water to produce acid rain and other hydrocarbons in the atmosphere to produce tropospheric ozone which ultimately adversely effects plant and animal life. These emissions can contribute to causing a number of severe health problems such as the chronic respiratory problems some human beings suffer today.

[0002] Because of the ongoing health concerns and the other adverse effects on plant and animal life the Environmental Protection Agency (EPA) has created many regulations to reduce emissions, and specifically NOx emissions. These EPA regulations continue to become more stringent, specifically on industry point sources such as furnaces.

[0003] Natural gas furnaces are the primary furnace/fuel types used in industry today. This is the case because the natural gas furnaces are utilized in a wide variety of applications in industry, and because natural gas is plentiful and a cleaner burning fuel. Because of the widespread use of this type furnace there is an ongoing effect to make sure that this type furnace complies with the regulations designed to reduce NOx emissions. As a result there is an ongoing search for inexpensive methods to maintain or gain compliance with EPA regulations on reducing emissions from natural gas furnaces.

BACKGROUND OF THE PRIOR ART

[0004] The technologies that have been utilized to reduce NOx emissions in furnaces fall into two categories, non-catalytic and catalytic.

[0005] One of the most popular catalytic techniques in industry is the ammonia injection technique. In this technique ammonia is reacted with the flue gas in the furnace. The ammonia reaction with the flue gas results in a reaction with the NOx to form diatomic nitrogen and water. This of course results in the reduction of NOx emissions from the furnace. Although this reaction results in reduction of NOx emissions, this technique can be relatively expensive to implement in that significant equipment additions and modifications are required.

[0006] Another technique for reducing NOx emissions is disclosed in U.S. Pat. No. 5,913,310. This technique for reducing NOx determines an NOx concentration profile within a zone of the furnace which is at a temperature below 2600° F. A stream of fluid fuel is then injected into at least one region of relatively high NOx concentration so that the fluid fuel mixes therein with the flue gas. The fluid fuel is natural gas, hydrogen, C_(x)H_(y) compounds, C_(x)H_(y)O_(z) compounds or mixtures primarily of those compounds, in sufficient quantity to promote a reaction between nitrogen oxide in the flue gas and the fluid fuel. This results in a substantial reduction of nitrogen oxide content of the flue gas.

[0007] This technique in most instances requires significant equipment additions and modifications, that are very investment intensive.

[0008] The non-catalytic techniques focus on two fundamental equilibrium principles to minimize NOx emissions. That is the reduction of excess oxygen content into the primary flame combustion zone and reduction of the adiabatic flame temperature.

[0009] One non-catalytic technique is known as the low excess air technique. This technique focuses on reducing excess oxygen content in the primary flame or combustion zone by lowering the excess air. This approach lessens the probability of diatomic nitrogen reacting with diatomic oxygen thus reducing NOx. This technique requires that proper instruments and flow controls are in place in the furnace. If proper instruments and flow controls are not in place, then capital investment will be required to implement this technique successfully.

[0010] Another non-catalytic technique is the Reduced Air Preheat technique. This technique is not widely utilized in the industry because most furnaces or boilers do not require feed preheat. Preheat is usually required when feed sources are very wet, and preheat is basically a drying step. Usually, preheaters operate between 200° F. and 500° F., and consequently, the adiabatic flame temperature will increase accordingly. The higher adiabatic flame temperature favors the NOx forming reactions, and is thus undesirable.

[0011] Still another technique is the flue gas re-circulation technique. In this technique, a slipstream of flue gas is recycled and mixed with primary air and fuel. The flue gas in this case is basically acting as a diluent, and will reduce the adiabatic flame temperature thereby reducing NOx emissions. The deficiency in this technique is that the furnace might not have the capacity to handle the additional flow required to implement this technique.

DISCLOSURE OF THE INVENTION

[0012] An improved combustion system for reducing NOx emissions in a furnace in accordance with the principles of this invention includes a primary flame zone for burning fuel which produces a flue gas containing no NOx and a secondary flame zone above the primary flame zone. A means is provided in the primary flame zone for facilitating the passage of uncombusted natural gas (or hydrocarbons) into the secondary flame zone. The combustion system is also provided with a means in the secondary flame zone for inducing a predetermined amount of air into the secondary flame zone so that uncombusted natural gas (or hydrocarbons) in the secondary flame zone is combusted thereby reducing emissions from the furnace.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The details of the invention will be described in connection with accompanying drawing in which:

[0014]FIG. 1 is a schematic of a furnace in accordance with the principles of the invention.

[0015]FIG. 2 is a side view of a furnace in accordance with the principles of the invention.

[0016]FIG. 3 is a schematic of a second embodiment of a furnace in accordance with the principles of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Referring to FIGS. 1 and 2, a natural gas furnace, generally designated by the numeral 10 is provided. The furnace 10 is provided with a radiant or primary flame zone, generally designated, by the numeral, 12. The primary flame zone 12 is provided with a plurality of natural gas burners 14 associated therewith in a well known manner. These burners 14 are designed in a well known manner so they can be operated at a sub-stoichiometric level by adjusting furnace ports 15. The ports 15 may be, for example, multiple ports formed in the primary flame zone so that air is induced therein in a well known manner to achieve desired sub-stoichiometric levels.

[0018] The furnace 10 is also provided with a convection zone 16 above the primary flame zone 12. In an area between the convection zone 16 and the primary flame zone is a secondary flame zone 18. The secondary flame zone 18 is provided with a natural draft port, generally designated, by the numeral, 20 formed in a wall 22 thereof The port 20 is formed in the wall 22 at a predetermined distance above the primary flame zone 12 and has a predetermined cross-sectional area. The port 20 is provided with an opening 23, and a door 24 coupled to the wall 22 in adjacent alignment with the opening 23 to allow the opening to be covered or uncovered as necessary. The opening 23 of the port 20 maybe, for example, 1.8 ft², maybe located between thirty (30) and thirty-one (31) feet above the primary flame zone 12. The port or ports are provided so that you can create mixing of induced air with uncombusted hydrocarbon which is required to ignite combustion. Although not shown here multiple ports 20 at predetermined positions may be used if desired.

[0019] The furnace 10 is also provided with a stack, generally designated, by the numeral, 26. The stack 26 is located directly above the convection zone 16. The furnace 10 maybe, for example, a 350 million BTU/hr heat duty furnace having a height of one-hundred (100) feet with a primary flame height of twenty feet, and a stack outlet temperature of 450°.

[0020] In the typical operation of the furnace 10 the fuel is burned in the primary flame zone 12 causing a flue gas of combustible products containing NOx to flow upward through the convection zone 16 and out through the stack 26 into the atmosphere undesirable NOx emissions.

[0021] The technique in accordance with the principles of this invention for reducing NOx emissions dictates that the furnace 10 be operated so that complete combustion isn't achieved in the primary flame zone 12. To achieve this, furnace ports in the primary flame zone are adjusted to a predetermined cross-sectional area to generate sub-stoichiometric air flow to the burners, such as, for example, a 0.9 sub-stoichiometric factor. This will produce a fuel gas stream through the primary flame zone 12 and into the secondary flame zone 18 with uncombusted natural gas (or hydrocarbons). In order for NOx emissions from the furnace to be reduced the uncombusted natural gas (or hydrocarbons) has to be combusted in the secondary flame zone 18. This is achieved by inducing air into the secondary flame zone 18 at the predetermined level above the primary flame zone 16 at a lower adiabatic flame temperature than that of the primary flame thereby combusting the remaining natural gas (or hydrocarbons). The natural draft through the port 20 of the secondary flame zone 18 causes air to be induced into the secondary flame zone. This natural draft and resulting air induction is caused because of the location of port 20 above the primary flame zone 12 and the size of the port. The natural draft created by the port 20 induces enough air into the secondary flame zone 18 to complete combustion, and thereby reduce undesirable NOx emissions.

[0022] The key to reducing NOx emissions in accordance with this invention is reducing excess oxygen content in the feed, typically contained in air, and reducing the adiabatic flame temperature in the secondary flame zone 18 of the furnace 10. To understand how this NOx reduction technique impacts excess oxygen content in the feed and the adiabatic flame temperature, combustion equilibrium equations are derived so the equilibrium reactions in the burner 14 can be understood. The following 2 reactions are assumed to occur in the burner 14:

CH₄+2O₂→CO₂+2H₂O  1)

N ₂+O₂→2NO.  2)

[0023] Reaction 1 is assumed to always react to completion. Reaction 2 is assumed to produce NOx as nitrous oxide because nitrous oxide makes up 95% of NOx. Ultimately, the nitrous oxide is converted into nitrogen dioxide in the atmosphere. Equilibrium equations were developed for the 2 reactions, and are illustrated below. For reaction 1 the following was developed:

K _(eq1)=[(c+x ₁)(d+2x ₁)]/[(a−x ₁)(b−2x ₁)]→2(k _(eq1)−1)x ₁ ²−[(2a+b)K _(eq1) +d+2c]x ₁ +abK _(eq1) −dc=0

[0024] This equation is simplified as follows:

A=2(k _(eq1)−1)

B=[(2a+b)K _(eq1) +d+2c]

C=abK _(eq1) −dc

[0025] Solving for x₁ which is the consumed methane, gives the following:

x ₁ ={−B+/−[(B ²−4AC)]^(1/2)}/2A

[0026] For the second reaction, the following equilibrium equation is generated:

K _(eq2)=(x ₂ +c)/[(a−1/2x ₂)(b−1/2x ₂)]→1/4K _(eq2) x ₂ ²−[1/2K _(eq2)(a+b)+1]x ₂ +abK _(eq2) −c=0

[0027] This equation is simplified as follows:

A=1/4K _(eq2)

B=−[1/2K _(eq2)(a+b)+1]

C=abK _(eq2) −c

[0028] Solving for x₂ which is produced nitrous oxide, gives the following:

x ₁ {−B+/−[(B ²−4AC)]^(1/2)}/2A

[0029] For the second reaction, the following equilibrium equation is generated:

Keq1=(x2+c)/[(a−1/2x2)(b−1/2x2)]→1/4K _(eq2) x ₂ ²−[1/2K _(eq2)(a+b)+1]x ₂ +abK _(eq2) −c=0

[0030] This equation is simplified as follows:

A=1/4K _(eq2)

B=−[1/2K _(eq2)(a+b)+1]

C=abK _(eq2) −c

[0031] Solving for x₂, which is produced nitrous oxide, gives the following:

x ₁ ={−B+/−[B ²−4AC)]^(1/2)}/2A

[0032] The equilibrium constant is a function of ΔG_(f), and ΔG_(f) is a function of temperature as shown below:

ΔG _(f) =R*T*ln(K _(eq)), and

ΔG _(f) =ΔH _(f) −T*ΔS _(f).

[0033] If ΔS_(f) is assumed to be constant, then the following equation is derived for the equilibrium constant:

K _(eq) =exp(ΔG _(o) /R*T _(o))*exp(1/R*(ΔH _(fo) /T _(o) −ΔH _(f) /T _(f1))).

[0034] The adiabatic flame temperature is (T_(f1)).

[0035] In order to determine the adiabatic flame temperature the material balance is qualified as a function of x_(i) (methane consumed) and x₂ (nitrous oxide produced). All other reactant and by-product components are calculated on a stoichiometric basis as follows: O₂ Initial O₂* excess air factor-2* x₁ − ½*x₂ N₂ 79/21*(Initial O₂*excess air factor) − ½*x₂ CO₂ Initial CO₂ + x₁ H₂O Initial H₂O + 2*x₁

[0036] The following table is an illustration of the material balance for the simulation generated from the above equations: Material balance, lb mole x₀ x₁ x₂ N₂ 11613.44 11613.44 11613.44 CH₄ 1102.54 0.00 0.00 O₂ 3087.12 882.03 882.03 CO₂ 0.00 1102.54 1102.54 H₂O 0.00 2205.08 2205.08 NOPPM 0.00 N/A 1386.25 TOTAL 15803.10 15803.10 15803.10

[0037] The energy balance is a function of the material balance.

[0038] The first step is to calculate the heat of reaction, which is qualified as follows:

Q reaction=ΔH _(f) x ₁, where

ΔH _(f) =ΔH _(f0) +∫Cp(T)dT.

[0039] The adiabatic flame temperature is calculated as follows:

dQ/dT=Cp(T)*m,

where Cp=A*T ² +B*T+C.

[0040] The integral is of the following form:

∫dQ =m*(A*T ² +B*T+C)dT.

[0041] When integrated from T₀ to T_(f1), the following equation is generated:

Q=m*[A*(T _(f1) ³ −T _(o) ³)/3+B*(T _(f1) ² −T ₀ ²)/2+C*(T _(f1) −T ₀)]

[0042] The adiabatic flame temperature cannot be readily obtained from this equation, so the model iterates on T_(f1) until the following equation coverages:

Qreaction/{m*[A*(T _(f1) ³ −T ₀ ³)/3+B*(T _(f1) ² −T ₀ ²)/2+C*(T _(f1) −T ₀]}=1.

[0043] As can be seen, the equilibrium constant and the adiabatic flame temperature are interdependent. Because of the interdependency, the model iterates on the adiabatic flame temperature until all governing equations converge. The following table is an illustration of the energy balance for the simulation generated from the above equations: Energy Balance BTU T₀₁ ° F. = 77.00 Eq. 1 DEL H_(fT0) = −345006.20 Eq. 2 DEL H_(fT0) = 77615.00 Eq. 1 DEL Q_(fT) = −35576126.78 Tfl. ° F. = 2673.44 Eq. 1 DEL H_(fT) = −322505.78 Eq. 2 DEL H_(fT) = 100115.42 Excess Air Factor = 1.40 Q/(m*Cp(T)) = 1.00

[0044] Quantifying the furnace temperature profile is very significant in applying the principles of this invention because NOx emission reduction is accomplished by reducing the adiabatic flame temperature. With this technique, the primary flame zone 12 is operated at sub-stoichiometric levels. The uncombusted hydrocarbons (in this case methane) react with oxygen injected at a lower temperature at the predetermined level above the primary flame zone 12. To understand exactly where oxygen as air should be injected above the primary flame zone 12, the furnace temperature profile is required.

[0045] Heat transfer on the flue gas side of the furnace 10 is a function of three components. Conduction, forced convection, and heat absorption transfer heat on the flue gas side of the furnace. The independent variable for each component is the height of the furnace, which will be labeled with “x”. From the energy balance the following ordinary differential equation (ODE) is generated:

k _(eff) d ² T/dx ² −ρC _(p) V _(x) dT/dx−q=0,

[0046] where k_(eff) is the flue gas effective thermal conductivity constant (k_(eff) is the appropriate nomenclature if flow is turbulent), ρ is the flue gas density, V_(x) is the flue gas velocity in the x direction, and q is the net heat flux. This energy balance ODE assumes plug flow. The solution for this ODE has the following form:

T=A*(exp(B*x−1)+C,

[0047] where A and C are derived from boundary condition evaluations, and B has the following form:

(ρC _(p) V _(x)−(ρC _(p) V _(x)−4*k _(eff) *q)^(1/2))/(2*k _(eff))

[0048] A and C are derived from the following boundary conditions

X=0,T=T _(f1) and x=∞, T=T _(ambient).

[0049] Solutions to these boundary conditions lead to the following for A and C:

A=T _(f1) −T _(ambient) at x=∞, and C=T _(f1) at x=0

[0050] Combining the boundary solutions with the general form solution, the following equation is derived:

T=T _(f1) −T _(ambient)*(exp((ρC _(p) V _(x)−(ρC _(p) V _(x)−4*k _(eff) *q)^(1/2))/(2*k _(eff))*x)−1)+T _(f1).

[0051] The pressure drop profile is also required for the application of the technique of this invention in a natural gas furnace 10. Since additional air is required in the secondary flame zone 18 to complete combustion the pressure drop is quantified to determine what driving force is necessary to induce enough natural draft into the secondary flame zone to complete the combustion.

[0052] The pressure drop is a function of the velocity profile in the x-direction, and the velocity profile is a function of the base cross-sectional area. Consequently, velocity in the x-profile is a function of the y-direction and the z-direction. This two dimensional flow profile generates the following partial differential equation (PDE) from Navier-Stokes equations:

μ(∂² V _(x)/∂_(y) ²+∂² V _(x)/∂_(z) ²)=−Δp/Δx.

[0053] This PDE is solved using the integral and differential methods of the second-order polynomial approximation of the ritz profile. The solution has the following form:

V _(x)(y,z)=(Y ² −z ²)*(Z ² −y ²)*(a ₀ +a ₁ *z ²)

[0054] The integral and differential methods are required because a₀ and a₁ must be determined. The second order polynomial solution is being used because of accuracy. The above equation is substituted into the integral form as follows: ${\int^{y}{\int^{z}{\left( {\frac{\partial^{2}V_{x}}{\partial y^{2}} + \frac{\partial^{2}V_{x}}{\partial z^{2}} + {\left( {\Delta \quad {p/\Delta}\quad x} \right)/\mu}} \right){z}{y}}}} = 0.$

[0055] The solution form is also substituted into the differential form, which is the governing equation derived from Navier-Stokes equations:

∂² V _(x)∂² _(y)+∂² V _(x)/∂⁸ z+ ₍ Δp/Δx)μ=0.

[0056] The solutions to these two methods can be used to solve for a_(o) and a₁. The equation for Vx is as follows

V _(x)(y,z)=(−Δp/Δx)μ*{1/4(1−z ² /Y ²)*(1−y ² /Z ²)/[2+1/5*(Y/z)²]}*{[5+2/5*(YZ)²]/[1+(Y/Z)²]+(z/Y)²}

[0057] The average velocity is quantified by integrating the previous equation with respect to both independent variables, and dividing that solution buy the base cross-sectional area. The final solution has the Hagan-Poiseuille form, with a different coefficient:

−₍ Δp/Δx)=63*μ*V _(xavg) /Y ²

[0058] After integration, the following equation is generated:

Δp=63*μ*V _(xavg) *L/Y ²

[0059] Since the stack average velocity is a function of diameter only, the solution is the same as the Hagen-Poiseulle form. However, the flow in the stack is turbulent, and the pressure drop is calculated as follows:

Δp/p=h _(L,)

h _(L) =f*V ² _(xavg) *L/(2*D)

and

f=0.3164/Re ^(0.25.)

[0060] which is the Blasius correlation for turbulent flow in a smooth pipe. Combining these equations generates the following equation for pressure drop in the stack:

Δp=0.1582*Re ^(0.75) *L*V _(xavg) *μ/D ²

[0061] As previously noted, the pressure drop profile is quantified to determine what driving force is necessary to induce enough natural draft to complete the combustion in the secondary flame zone. The following equation shows the relationship between pressure drop and the natural draft air rate:

m_(air)=0.65*A[2*g _(c) *Δp/ρ] ⁰ ⁵*[14.7/(10.73*T _(amb.))],

[0062] where A is the cross-sectional area of the natural draft port for air flow.

[0063] In this invention air is induced into the secondary flame zone 18 at a defined point above the primary flame 12 through a natural draft port 20 formed in the side of the furnace wall 22 or through multiple ports (not shown) formed in the wall 22 and a wall opposed to the wall 22. The port has a 20 cross-sectional area large enough to generate the appropriate air flow from natural draft to complete the combustion in the secondary flame zone 18. Approximately 1500 lb.-mole/hr of air is required (1.4 excess air factor) to complete the combustion in the secondary flame zone 18 where the primary flame zone 12 is operated at 0.9 sub-stoichiometric factor. This is accomplished via a natural draft a port size of 1.8 ft² located in the wall 22 of the secondary flame zone 18 between thirty (30) and thirty-one (31) feet above the primary flame zone 12. If multiple ports were used, there may be, for example, four ports, one on each wall that are {fraction (1/4)} the size of the 1.8 ft² port. By inducing air in the secondary flame zone 18 between thirty (30) and thirty-one (31) feet above the primary flame zone 12 the NOx emissions are reduced well within EPA requirements.

[0064] A second embodiment of the invention is illustrated in FIG. 3. In this embodiment of the invention a natural gas furnace, generally designated by the numeral 40 is provided. The furnace 40 is provided with a primary flame zone, generally designated, by the numeral, 42. The primary flame zone 42 is provided with a plurality of burners 44 associated therewith in a well known manner. The burners 44, just as burners 14 in the first embodiment are designed to be operated at sub-stoichiometric levels by adjusting furnace ports 45. The ports 45 may be multiple ports formed in the primary flame zone so that natural gas is induced therein to achieve desired sub-stoichiometric levels.

[0065] The furnace 40 is also provided with a convection zone 46 above the primary flame zone 42. In an area between the convection zone 46 and the primary flame zone 42 is a secondary flame zone 48. The secondary flame zone 48 is provided with an air injector 50 formed in a wall 52 of the secondary flame zone. The injector 50 is formed in the wall 52 at a predetermined distance above the primary flame. This distance is the same distance as is set in the first embodiment with regard to the port 20. The injector 50 is provided to inject the same quantities of air into the secondary flame zone 48 as is illustrated in the first embodiment with regard to the port 20. Forced draft from the injector 50 can be accomplished by installing, for example, a blower or fan with the appropriate capacity at the designated distance above the primary flame zone 42. The furnace 40 is also provided with a stack, generally designated, by the numeral, 56 which is located directly above the convection zone 46. In this embodiment of the invention, 1500 lb.-mole/hr of air is injected into the secondary flame zone 48 of the furnace 40. The air is injected just as with the first embodiment in the secondary flame zone at between thirty (30) and thirty-one (31) feet above the primary flame zone. This permits NOx emission reduction well within EPA requirements. It should be understood that principles of this invention may be applied to furnaces of different sizes and types without departing from the spirit and scope of the invention.

[0066] It should be further understood that various changes and modifications can be made to the invention without departing from the spirit of the invention as defined in the claims. 

What is claimed:
 1. An improved combustion system for reducing NOx in a furnace including: a primary flame zone for burning fuel which produces flue gas without NOx; a secondary flame zone above the primary flame zone; means in the primary flame zone for facilitating the passage of uncombusted natural gas into the secondary flame zone; and means in the secondary flame zone for inducing a predetermined amount of air into the secondary flame zone so that uncombusted natural gas in the secondary flame zone is combusted thereby reducing NOx emission from the furnace.
 2. An improved combustion system as defined in claim 1 wherein the means for facilitating the passage of uncombusted natural gas into the secondary flame zone includes a plurality of burners which are activated so that the burners are operated at sub-stoichiometric levels.
 3. An improved combustion system as defined in claim 2 wherein the burners are operated at a 0.9 sub-stoichiometric factor.
 4. An improved combustion system as defined in claim 3 wherein the means in the secondary flame zone for inducing a predetermined amount of air into the secondary flame zone includes a port having a predetermined cross-sectional area, formed at a predetermined distance above the primary flame zone.
 5. An improved combustion system as defined in claim 4 wherein the port includes a door aligned adjacent to the aperture to facilitate the opening and closing of the aperture.
 6. An improved combustion system as defined in claim 5 wherein the aperture has a cross-sectional area of one and eight-tenths square feet.
 7. An improved combustion system as defined in claim 6 wherein the aperture is aligned in the secondary flame zone thirty feet above the primary flame zone.
 8. An improved combustion system as defined in claim 7 wherein the port facilitates the induction of fifteen hundred lb.-mole per hour of air into the secondary flame zone to complete combustion of natural gas therein.
 9. A method of reducing NOx emissions in a furnace including the steps of: operating a primary flame zone of the furnace so that uncombusted natural gas passes into a secondary flame zone of the furnace; and inducing a predetermined amount of air into the secondary flame zone at a predetermined level above the primary flame zone so that the uncombusted natural gas is combusted thereby reducing NOx emissions from the furnace. 10 A method for reducing NOx emissions in a furnace as defined in claim 9 wherein the primary flame zone is operated at sub-stoichiometric levels.
 11. A method for reducing NOx emissions in a furnace as defined in claim 10 wherein the primary flame zone is operated at a 0.9 sub-stoichiometric factor.
 12. A method for reducing NOx emissions in a furnace as defined in claim 11 wherein the air induced into the secondary flame zone is thirty feet above the primary flame zone.
 13. A method for reducing NOx emissions in a furnace as defined in claim 12 wherein the induction of air into the secondary flame zone is facilitated by an aperture formed therein having a cross-sectional area of one and eight-tenth square feet.
 14. A method for reducing NOx emissions in a furnace as defined in claim 13 wherein fifteen hundred lb.-mole per hour of air is induced into the secondary flame zone.
 15. An improved furnace including: a primary flame zone for burning fuel which produces flue gas without NOx; a secondary flame zone above the primary flame zone; means in the primary flame zone for facilitating the passage of uncombusted natural gas into the secondary flame zone; and means in the secondary flame zone for inducing a predetermined amount of air into the secondary flame zone so that uncombusted natural gas in the secondary flame zone is combusted thereby reducing NOx emissions from the furnace.
 16. An improved combustion system as defined in claim 15 wherein the means for facilitating the passage of uncombusted natural gas into the secondary flame zone includes a plurality of burners which are activated so that the burners are operated at sub-stoichiometric levels.
 17. An improved combustion system as defined in claim 16 wherein the burners are operated at a 0.9 sub-stoichiometric factor.
 18. An improved combustion system as defined in claim 17 wherein the means in the secondary flame zone for inducing a predetermined amount of air into the secondary flame zone includes a port having an aperture having a predetermined cross-sectional area formed at a predetermined distance above the primary flame zone.
 19. An improved combustion system as defined in claim 18 wherein the port includes a door aligned adjacent to the aperture to facilitate the opening and closing of the aperture.
 20. An improved combustion system as defined in claim 19 wherein the aperture has a cross-sectional area of one and eight-tenths square feet.
 21. An improved combustion system as defined in claim 20 wherein the aperture is aligned in the secondary flame zone thirty feet above the primary flame zone.
 22. An improved combustion system as defined in claim 21 wherein the port facilitates the induction of fifteen hundred lb.-mole per hour of air into the secondary flame zone to complete combustion of NOx therein.
 23. An improved combustion system for reducing NOx in a furnace including: a primary flame zone for burning fuel which produces flue gas without NOx; a secondary flame zone above the primary flame zone; means in the primary flame zone for facilitating the passage of uncombusted natural gas into the secondary flame zone; and means in the secondary flame zone for injecting a predetermined amount of air into the secondary flame zone so that uncombusted natural gas in the secondary flame zone is combusted thereby reducing NOx emissions from the furnace.
 24. An improved combustion system as defined in claim 23 wherein the means for facilitating the passage of uncombusted natural gas into the secondary flame zone includes a plurality of burners which are activated so that the burners are operated at sub-stoichiometric levels.
 25. An improved combustion system as defined in claim 24 wherein the burners are operated at a 0.9 sub-stoichiometric factor.
 26. An improved combustion system as defined in claim 25 wherein the means in the secondary flame zone for injecting a predetermined amount of air into the secondary flame zone includes an injector formed therein at a predetermined distance above the primary flame zone.
 27. An improved combustion system as defined in claim 26 wherein the injector is aligned in the secondary flame zone thirty feet above the primary flame zone.
 28. An improved combustion system as defined in claim 27 wherein the injector facilitates the injection of fifteen hundred lb.-mole per hour of air into the secondary flame zone to complete combustion of natural gas therein.
 29. A method of reducing NOx emissions in a furnace including the steps of: operating a primary flame zone of the furnace so that uncombusted natural gas passes into a secondary flame zone of the furnace; and injecting a predetermined amount of air into the secondary flame zone at a predetermined level above the primary flame zone so that the uncombusted natural gas is combusted thereby reducing NOx emissions from the furnace.
 30. A method for reducing NOx emissions in a furnace as defined in claim 29 wherein the primary flame zone is operated at sub-stoichiometric levels.
 31. A method for reducing NOx emissions in a furnace as defined in claim 30 wherein the primary flame zone is operated at a 0.9 sub-stoichiometric factor.
 32. A method for reducing NOx emissions in a furnace as defined in claim 31 wherein the air injected into the secondary flame zone is thirty feet above the primary flame zone.
 33. A method for reducing NOx emissions in a furnace as defined in claim 32 wherein fifteen hundred lb.-mole per hour of air is injected into the secondary flame zone. 